Mysql multi-table query detailed explanation

Mysql multi-table query detailed explanation

I. Introduction

2. Example

3. Matters needing attention

I. Introduction

The last part talked about the order of keyword execution in Mysql, only one table is involved; in most cases of practical applications, the query statement will involve multiple tables:

1.1 What are the categories of multi-table joins ?

1.2 What connection methods are available for these categories ?

1.3 Which application scenarios are these connection methods used for ?

This article describes these three points through examples, the purpose is to exhaust all scenarios and all methods, and to give examples of the use of each method.

First, list the classifications (inner joins, outer joins, cross joins) and connection methods used in this article (as follows):

A) Inner join : join, inner join

B）外连接：left join，left outer join，right join，right outer join，union

C) Cross join : cross join

2. The following is an example of analysis

Two Suppose there are two tables A and B, and the table is regarded as a set, then the record in the table is an element in the set.

The two tables are as follows:

TableA：TableB：

2.1 Inner connection (only one scenario)

inner join or join (equivalent to inner join)

[java] view plain copy

select a.*, b.* from tablea a
inner join tableb b
on a.id = b.id

[java] view plain copy

select a.*, b.* from tablea a
join tableb b
on a.id = b.id

The result is as follows:

Application Scenario:

In this scenario, the data inside A and B that meet a certain condition is obtained; just because the obtained data is shared internally, the connection method is called inner connection.

2.2 Outer join (six scenarios)

2.2.1 left join 或者left outer join(等同于left join)

[java] view plain copy

select a.*, b.* from tablea a
left join tableb b
on a.id = b.id

[java] view plain copy

select a.*, b.* from tablea a
left outer join tableb b
on a.id = b.id

The result is as follows, with more non-existent records in TableB filled with Null:

Application Scenario:

In this scenario, all the data of A and the data of B that meet a certain condition are obtained;

2.2.2 [left join 或者left outer join(等同于left join)] + [where B.column is null]

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
left join tableb b
on a.id = b.id
Where b.id is null

The result is as follows:

Application Scenario:

In this scenario, all the data in A minus "data that meets the same conditions as B", and then the remaining data of A is obtained;

2.2.3 right join or fight outer join (equivalent to right join)

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
right join tableb b
on a.id = b.id

The result is as follows, with more non-existent records in TableB filled with Null:

Application Scenario:

In this scenario, all the data of B and the data of A that meet a certain condition are obtained;

2.2.4 [left join 或者left outer join(等同于left join)] + [where A.column is null]

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
right join tableb b
on a.id = b.id
where a.id is null

The result is as follows:

Application Scenario:

What is obtained in this scenario is all the data in B minus "data that meets the same conditions as A", and then the remaining data of B is obtained;

2.2.5 full join （mysql不支持，但是可以用 left join union right join代替）

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
left join tableb b
on a.id = b.id
union
select a.id aid,a.age,b.id bid,b.name from tablea a
right join tableb b
on a.id = b.id

union过后，重复的记录会合并（id为2，3，4的三条记录），所以结果如下：

应用场景：

这种场景下得到的是满足某一条件的公共记录，和独有的记录

2.2.6 full join + is null（mysql不支持，但是可以用（left join + is null） union （right join+isnull代替）

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
left join tableb b
on a.id = b.id
where b.id is null
union
select a.id aid,a.age,b.id bid,b.name from tablea a
right join tableb b
on a.id = b.id
where a.id is null

结果如下:

应用场景:

这种场景下得到的是A，B中不满足某一条件的记录之和

注:上面共有其中七（2^3-1）种应用场景，还有一种是全空白，那就是什么都不查，七种情形包含了实际应用所有可能的场景

2.3 交叉连接（cross join）

2.3.1 实际应用中还有这样一种情形，想得到A，B记录的排列组合，即笛卡儿积，这个就不好用集合和元素来表示了。需要用到cross join：

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
cross join tableb b

2.3.2 还可以为cross join指定条件（where）：

[java] view plain copy

select a.id aid,a.age,b.id bid,b.name from tablea a
cross join tableb b
where a.id = b.id

结果如下;

注:这种情况下实际上实现了内连接的效果

三注意事项

There are still omissions above, that is, the fault tolerance of mysql for sql statements, that is, in the case that sql statements do not fully meet the writing recommendations, mysql will allow this situation and explain it as much as possible:

3.1 Generally, the where condition is added after the cross join, but the use of cross join+on is also interpreted as cross join+where;

3.2 In general, the on qualification needs to be added to the inner connection, such as the above scenario 2.1; if it is not added, it will be interpreted as a cross connection;

3.3 If a comma is used in the connection table, it will be interpreted as a cross connection;

Note: There are also union join and natural inner join in the SQL standard. MySQL does not support it, and it does not make much sense. The results can be obtained by using the above connection methods.

Summary: Summarize all connection methods of mysql, some of which are problems that have not been noticed before, and the usual development is nothing more than these.

PS-1: Since the previous typesetting was not beautiful enough, this article has been reorganized in order to better learn and communicate for everyone

Copyright statement: This article is an original article by the blogger and may not be reproduced without the blogger's permission. https://blog.csdn.net/Jintao_Ma/article/details/51260458

Mysql multi-table query detailed explanation

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